Userspace namespaces/chroots/jails are NOT virtualizations, period. Plan9/9front it's composed/run on namespaces and no one would say every window(1) process it's being virtualized because it has instances of different Plan9 devices such as their own /dev/draw.
https://en.wikipedia.org/wiki/OS-level_virtualization
Good to know about it. Especially once you meet decades-experienced folks (mainframe guys), they rather use the word "virtualization" in its broader sense, yet they understand the difference between containers and virtual machines.